1. Field of the Invention
This invention relates to a novel granulated nitrogen-phosphorus-potassium-sulfur fertilizer. More particularly, this invention relates to such fertilizer which is prepared from phospho-gypsum slurry by-product from the wet acid process of manufacturing phosphoric acid from phosphate rock or fluoroapatite.
2. Description of Prior Art
Natural phosphate rock, or fluorapatite, is the primary source of phosphoric acid. Two processes are in common use around the world to produce phosphoric acid. One process is the electric furnace process, and the other process is the acid, or wet, process.
The acid or wet process uses a strong mineral acid, usually sulfuric acid, to digest the rock, thereby releasing phosphoric acid. This digestion step also produces a residue of calcium sulfate, or phospho-gypsum, and small quantities of phosphorus, fluorine and various trace elements. This phospho-gypsum by-product, at present, has no commercial value and, in fact, poses an environmental problem because of the contamination of rainwater run-off by soluble compounds in the phospho-gypsum.
Prior to this invention, there have been many attempts to devise processes to recover all or part of the economically recoverable products contained in the waste phospho-gypsum and to concurrently solve the waste disposal problem. In Europe and other parts of the world, the phospho-gypsum is disposed of by dumping in the open sea. The readily apparent disadvantage of this method of disposal is that it results in loss of all commercial value. In other areas, notably the United States, the phospho-gypsum is disposed of by storage in a storage pile or impounding basin. The storage method of disposal also suffers from a number of inherent disadvantages. For example, the leaching action of rainwater and/or storage water produces an acidic effluent that may enter the nearby surface and/or groundwater regime, creating an environmental hazard. In recent years the practical impact of this disadvantage has become more acute in that increasingly stringent regulations require the collection of the effluent waters and neutralization. This collection and neutralization process is an expensive and nonproductive process.
The technical literature of the last 40 years or more includes discussions of and patents for various methods for the commercial use of all or part of the constituents of phospho-gypsum. In Japan, the United Kingdom, and several other countries, where natural gypsum is in short supply, the phospho-gypsum has been economically converted to plaster products, such as Plaster of Paris, to gypsum wallboard, or as an additive to portland cement, acting as a set retarder. This is not economically feasible in places such as North America where natural gypsum abounds.
Various proposals have been made for converting phospho-gypsum into useful and economical products by chemical means. In every instance, although technically feasible, the cost of the chemicals required to carry out the conversion has been greater than the value of the resulting product. An example is the reaction of gypsum with ammonia and carbon dioxide to form ammonium sulfate and calcium carbonate. Because of its low purity as compared to natural gypsum, the use of phospho-gypsum has not proven economical in this manner.